1. Field of the Invention
The present invention relates to a printed circuit board unit including stacked solder or conductive bumps between a printed wiring substrate and a mounted component, and particularly, to an interposer or relay substrate interposed between the stacked conductive bumps.
2. Description of the Prior Art
A printed circuit board unit is well known to include a mounted component such as a ball grid array (BGA) device, for example. When the electric circuit generates heat in the printed circuit board unit, a heat or thermal stress is in general induced in the printed wiring substrate, made of a glass epoxy or polyimide resin, and the ceramic substrate of the BGA device. Since the printed wiring substrate made of resin and the ceramic substrate have different thermal expansion coefficients, the substrates usually suffer from a relative movement or shift along the surface of the printed wiring substrate due to the thermal expansion.
Such a relative movement between the printed wiring substrate made of resin and the ceramic substrate induces a shearing stress in solder bumps disposed between the printed wiring substrate and the ceramic substrate. The shearing stress may repeatedly be generated in the solder bumps in response to switching between on and off statuses of the electric circuit in the printed circuit board unit. The solder bumps may sometimes suffer from cracks when the shearing strain finally overcomes the strength of the solder bumps. The thus repeated and intermittent generation of the shearing stress is supposed to deteriorate the durability of the solder bumps.
It is conventionally known that a higher or taller solder bump leads to reduction in the shearing stress induced in the solder bump. A higher solder bump contributes to a broader distribution of the shearing stress, so that a smaller shearing stress can be defined within a horizontal cross-section of the solder bump. However, an increase in the height of a spherical solder bump or solder ball inevitably induces an increase in the width of the solder ball. A higher or taller solder ball in this manner is supposed to suffer from a smaller distribution density of the solder balls.
In view of the above-described disadvantage, it is proposed that solder bumps are vertically stacked on the printed wiring substrate below the mounted component. The stacked solder bumps are supposed to realize an increase in the height without increasing the width. In this proposal, the solder bumps and relay substrates are alternately stacked on one another on the printed wiring substrate. The relay substrate serves to connect the adjacent stacked solder bumps.
As disclosed in Japanese Patent Application Laid-open No. 09-214088, for example, it is proposed that the thermal expansion coefficient of the relay substrate is set at an intermediate level between the thermal expansion coefficients of the printed wiring substrate and the mounted component. In this case, a shearing stress is equally distributed over the stacked solder bumps. Japanese Patent Application Laid-open No. 62-18049 still proposes to interpose a relay substrate in the form of a film having a smaller Young""s modulus between the stacked solder bumps. The relay substrate is supposed to absorb a larger shearing stress.
It is accordingly an object of the present invention to provide an interposer or a relay substrate for a multileveled printed circuit board unit capable of more efficiently absorbing a repetitive shearing stress induced in conductive bumps stacked between a printed wiring substrate and a mounted component, and a printed circuit board unit employing the same interposer.
It is another object of the present invention to provide an interposer or relay substrate capable of introducing additional functions in a multileveled printed circuit board unit.
According to a first aspect of the present invention, there is provided an interposer for a multileveled printed circuit board unit in which conductive bumps are stacked between a printed wiring substrate and a mounted component, comprising an interposer substrate made of a porous material interposed between stacked conductive bumps.
In the multileveled printed circuit board unit employing the interposer, when any difference in the expansion is caused between the printed wiring substrate and the mounted component, one side of the interposer substrate receives a relatively smaller displacement force while the other side of the interposer substrate receives a relatively larger displacement force. A shearing stress is induced in the interposer substrate. Deformation of the porous material serves to absorb the shearing stress in the interposer substrate. The conductive bumps bonded on one side of the interposer substrate as well as the conductive bumps bonded on the other side of the interposer substrate may be relieved from a shearing stress. Accordingly, the durability of the conductive bumps can be improved. The conductive bumps are allowed to keep a stronger bonding in a longer duration.
The porous material is preferably designed to define pores penetrating through the interposer substrate. The pore may be utilized to provide a via establishing an electric connection between the conductive bumps on the opposite sides. It is not necessary to bore the interposer substrate solely for the via. Production process of the interposer substrate can be facilitated.
The pore is designed as a slit located between adjacent ones of the conductive bumps arranged on an identical level. Even when one side of the interposer substrate receives a relatively smaller displacement force while the other side of the interposer substrate receives a relatively larger displacement force in the aforementioned manner, a shearing stress can be absorbed by deformation of the slit. In this case, the size of the slit at a central area of the interposer substrate is preferably set smaller than that of the slit located at a area remoter from the central area, since the planar displacement or shift gets larger at a location remoter from the center of the interposer substrate upon a thermal expansion of the printed wiring substrate and the mounted component.
According to a second aspect of the present invention, there is provided an interposer for a multileveled printed circuit board unit in which conductive bumps are stacked between a printed wiring substrate and a mounted component, comprising multilayered films made of different materials interposed between stacked conductive bumps.
It is possible to provide different characteristics at the opposite sides of the interposer substrate, respectively. For example, when the film contacting the mounted component may be made of a material having a thermal expansion coefficient identical to that of the mounted component, no shearing stress is induced in the conductive bumps between the mounted component and the film. On the other hand, when the film contacting the printed wiring substrate may be made of a material having a thermal expansion coefficient identical that of the printed wiring substrate, no shearing stress is likewise induced in the conductive bumps between the film and the printed wiring substrate. The material of the film contacting the mounted component may be selected from inorganic materials forming the substrate of the mounted component. The material of the film contacting the printed wiring substrate is selected from organic materials forming the printed wiring substrate. A relative sliding movement between the films may be accomplished by an adhesive layer interposed between the films. The adhesive layer may be made of an adhesive for coupling the films with each other.
According to a third aspect of the present invention, there is provided an interposer for a multileveled printed circuit board unit, comprising: a first conductive bump on a first level; and a second conductive bump on a second level stacked on the first conductive bump between a printed wiring substrate and a mounted component, said second conductive bump having a size different from that of the first conductive bump.
In general, the larger the size or diameter of a conductive bump gets, the larger the strength of the conductive bump can be obtained. The first and second conductive bumps are allowed to provide different strengths optimally required in the separate levels of the stacked conductive bumps, so that it is possible to minimize the size or diameter of the first and second conductive bumps. The overall height of the stacked solder bumps can thus be reduced, keeping a required strength. Alternatively, the first and second conductive bumps may be made of different materials, so that diffusion bonding of various strengths can be obtained between the conductive bumps and the interposer substrates as well as the other substrates, respectively.
According to a fourth aspect of the present invention, there is provided an interposer for a multileveled printed circuit board unit in which conductive bumps are stacked between a printed wiring substrate and a mounted component, comprising: an interposer substrate interposed between stacked conductive bumps; and a conductive wiring pattern formed on the interposer substrate so as to electrically connect at least the conductive bumps arranged on an identical level.
In the multileveled printed circuit board unit employing the interposer, the conductive wiring pattern on the interposer substrate serves to establish an alternative electric path for a disconnected wiring pattern on the printed wiring substrate and/or the mounted component. Also, the conductive wiring pattern may establish a modified electric path in place of a wiring pattern initially formed on the printed wiring substrate and/or the mounted component. In this case, two or more components may be mounted on the interposer substrate. In such a printed circuit board unit, the conductive wiring pattern on the interposer substrate may serve to establish an alternative electric path for a disconnected wiring pattern on the printed wiring substrate so as to electrically connect the mounted components to each other. Otherwise, the conductive wiring pattern may establish a modified electric path for a wiring pattern initially formed on the printed wiring substrate so as to connect the mounted components to each other.
According to a fifth aspect of the present invention, there is provided an interposer for a multileveled printed circuit board unit in which conductive bumps are stacked between a printed wiring substrate and a mounted component, comprising: an interposer substrate interposed between stacked conductive bumps; and a conductive pad disposed at a periphery of an area receiving the conductive bumps on the interposer substrate so as to receive a connecting wire.
In the multileveled printed circuit board unit employing the interposer, the connecting wire can simply be employed to establish an electric connection between the mounted components. In addition, since the conductive pads are allowed to receive the connecting wire at the periphery of the area receiving the conductive bumps on the interposer substrate, it is possible to easily fix the connecting wire to the target conductive pad without disassembling the multileveled printed circuit board unit.
According to a sixth aspect of the present invention, there is provided an interposer for a multileveled printed circuit board unit in which conductive bumps are stacked between a printed wiring substrate and a mounted component, comprising: an interposer substrate interposed between stacked conductive bumps; and a conductive pad disposed at a periphery of an area receiving the conductive bumps on the interposer substrate so as to receive a probe.
In the multileveled printed circuit board unit employing the interposer, it is possible to easily conduct an inspection for electric connection between the mounted component and the printed wiring substrate by simply contacting the tip ends of inspection probes with the target conductive pads. The inspection may serve to reveal a disconnection of the wiring pattern formed on the printed wiring substrate, a disconnection of the wiring pattern within the mounted component, and a disconnection at the respective conductive bumps. Moreover, since the conductive pads are allowed to receive the inspection probe at the periphery of the area receiving the conductive bumps on the interposer substrate, it is possible to easily contact the inspection probe with the target conductive pad without disassembling the multileveled printed circuit board unit.
According to a seventh aspect of the present invention, there is provided an interposer for a multileveled printed circuit board unit in which conductive bumps are stacked between a printed wiring substrate and a mounted component, comprising: an interposer substrate interposed between stacked conductive bumps; and a heat radiator attached to the interposer substrate.
In the multileveled printed circuit board unit employing the interposer, when the electric circuit operates to generate heat in the interposer substrate, heat radiation from the interposer substrate can be promoted through the heat radiator. The heat radiator may take the form of a fin, a thermal conductive pattern formed on the surface of the interposer substrate, and the like.
According to an eighth aspect of the present invention, there is provided an interposer for a multileveled printed circuit board unit in which conductive bumps are stacked between a printed wiring substrate and a mounted component, comprising: an interposer substrate interposed between stacked conductive bumps; and a standoff member standing on the interposer substrate so as to receive at least one of the printed wiring substrate and the mounted component at its tip end.
Employment of the interposer enables a method of mounting a component on a printed wiring substrate, comprising: preparing an interposer including a standoff member standing on an interposer substrate; preparing conductive bumps stacked between the printed wiring substrate and the component while allowing the interposer interposed between stacked conductive bumps; and reflowing the conductive bumps while the interposer and the component are urged against the printed wiring substrate. In this method, a pressure during a reflowing process enables leveling the height of the resulting conducive bumps, regulated by the height of the standoff member, on the identical level even if an irregularity can be found in the height of the conductive bumps on the identical level before ref lowing. The respective conductive bumps reliably achieve a bonding between the mounted component and interposer substrate as well as between the interposer substrate and the printed wiring substrate.
According to a ninth aspect of the present invention, there is provided an interposer for a multileveled printed circuit board unit in which conductive bumps are stacked between a printed wiring substrate and a mounted component, comprising: an interposer substrate interposed between stacked conductive bumps; and a guide member located on the interposer substrate at a predetermined location for adjustment of a relative position between the printed wiring substrate and the mounted component.
The guide member serves to prevent a displacement or slippage of the mounted component relative to the printed wiring substrate even when the conductive bumps are caused to melt during a reflowing process. The conductive bumps can reliably be bonded to the corresponding electrode pads on the mounted component, the interposer substrate and the printed wiring substrate, respectively.
The aforementioned interposers can be applied to a multileveled printed circuit board unit comprising: a printed wiring substrate: a component to be mounted on the printed wiring substrate; and conductive bumps stacked between the printed wiring substrate and the mounted component. It should be noted that the mounted component may include a ball grid array (BGA) device, a pin grid array (PGA) device, a flip chip, a chip for a multichip module (MCM), and the like.